1. Field of the Invention
This invention relates to the production of memory discs. More particularly, this invention relates to the production of very flat and smooth memory discs useful as information storage devices for computers or the like.
2. Description of the Prior Art
In the conventional production of a metal memory disc such as a so called "hard disc" useful for mass storage of data in the computer industry, an initial metal blank is stamped from a metal strip, mechanically machined, and then coated with a material containing magnetizable particles such as iron oxide. It is very important that the final product be very flat and smooth when used for such a purpose since read-write heads used with such storage devices are positioned so close to the disc that even the presence of smoke particles can result in an interference between the disc and the head resulting in a so-called "head crash".
Ottman et al. U.S. Pat. No. 4,393,628 discusses this need for a very flat and smooth disc and conclude that the conventional diamond machining of the disc is not sufficient to provide the needed smooth surface. The patentees point out that such machining, while providing a mirror-like finish, does include topography having a maximum peak to valley dimension that is 10 to 20 percent of the thickness of some coatings subsequently applied to the disc. They further observe that such variations in topography may be as much as 20 to 40 percent of some proposed coatings which could result in unacceptable localized thinning of the subsequent coating. A polishing step is therefore proposed to supplement the initial machining step.
Vernam et al. U.S. Pat. No. 4,412,870, assigned to the assignee of this invention, recognize that some difficulties with the machining of such discs may be the result of the presence of insoluble alloy constituents such as Al-Fe-Mn-Si phases in an aluminum base alloy. The patentees describe and claim an improved disc using a new aluminum wrought alloy and methods for making the alloy and the disc to provide a more machinable product.
However, it has been recognized that it would be economically advantageous to eliminate or minimize the surface machining step entirely, or at least restrict such to merely machining of the edges to maintain the diameter integrity of the disc, relying mainly on pressure steps for the desired flatness and smoothness of the disc.
An alternate approach proposed in European patent application Nos. 92,325 and 107,421 involves stretching a nonmetallic recording film across a rigid disc or substrate and relying on the created tension to provide the desired flatness.
The use of nonmetallic recording discs has also been shown in U.S. Pat. Nos. 3,554,798, 4,374,795 and 4,435,343 wherein the use of materials such as plastic are used. However, the use of such materials, particularly in applications such as so-called "floppy discs", has shown that such materials are not as durable as coated metal discs.